1. Field of the Invention
The present invention relates to organic light-emitting devices, and more particularly, to a white organic light-emitting device capable of effectively increasing the luminous efficiency, lowering operating voltages, and providing color stability.
2. Description of Related Art
Light-emitting layer of phosphorescent organic light-emitting devices generally achieves goals likes increasing luminous efficiency and adjusting photochromes by doping guest dopant with high quantum efficiency into host. Usually, the guest dopant is composed of organic complexes having transition metals.
To develop highly efficient blue PHOLEDs, a large triplet excited state of >2.7 eV for the host material is essential in order to prevent quenching of the dopant emission. (see Ref. Appl. Phys. Lett., 83, 569 (2003)) However, host materials that have large triplet excited state, in general, have wider bandgap which acts as energy barriers for the transport of carriers from nearby hole or electron-transporting layer to phosphorescent emitter. Consequently, blue PHOLEDs tend to have higher driving voltage (>6 V) and lower power efficiency (<10 lm/W).
Fabrication of white organic light-emitting devices can be achieved by admixing more than two guest dopants with different emission wavelength (such as blue/yellow, blue/red or blue/green/red) in a single host material, and then adjusting respective concentrations of the guest dopant to generate white-light. However, this method has the drawback in the difficulty in controlling concentrations of respective host materials, which is adverse to fabrication.
Therefore, two or more light-emitting layers with different photochromes are typically used in the industry for fabrication of a white organic light-emitting device, wherein a blue light-emitting layer is essential in the fabrication. Nevertheless, in the context of a device having a light-emitting multilayer, when different operating voltages are applied, the color is changed due to the shift of carrier recombination zone. To solve the above-mentioned problems, TW I279165 and US 2007/0035240 disclose a blue light-emitting layer that is disposed between two red light-emitting layers, wherein the two red light-emitting layers include the same host material, guest dopant and thickness of the layers, so as to suppress color deviation. JP 2007/335214 discloses an organic light-emitting device in which a light-emitting layer with a long emission wavelength (yellow or red) is disposed between two light-emitting layers with short emission wavelength (blue or green), wherein the compositions of the two light-emitting layers with short emission wavelength are the same. JP 2007/189002 discloses an organic light-emitting device in which a yellow/red light-emitting layer is disposed between two blue light-emitting layers, wherein the two blue light-emitting layers include different guest materials, and the guest materials are defined, respectively, as “a material having an emission wavelength ranging from 430 to 465 nm” and “a material having an emission wavelength ranging from 465 to 485 nm.”
However, when a white organic light-emitting device comprises a light-emitting multilayer in which layers are composed of the same host material, a barrier for transport or injection of charge carriers between adjacent hole transport layer and/or electron transport layer is generated, thereby causing problems of high operating voltage and low luminous efficiency as described above.